BACKGROUND: Can vaccination with Bacille Calmette-Guérin prevent clinical progression of COVID-19? Data from the BCG-REVAC trial was archived in a database, creating an excellent opportunity to link it to notified cases of COVID-19 to evaluate the efficacy of BCG against incidence, severity and clinical progression to severe COVID-19 when given at birth day, at school age as a first dose or as a second dose. METHODS: This study was conducted in the population of the BCG-REVAC cluster randomisation trial including 354,403 schoolchildren, aged 7 to 14 years, from 767 schools from two cities, Salvador and Manaus. Cases of COVID-19 from the System for Notification of Infectious Diseases and the System for Notification of Severe Respiratory Illnesses were record linked to BCG-REVAC population. The exposure was Vaccination or revaccination obtained by the BCG-REVAC. The outcomes of interest in this study were incidence COVID-19; incidence of severe COVID-19; and clinical progression of COVID-19. This project was approved by the Ethics Committee of the Institute of Collective Health, Federal University of Bahia, Brazil. RESULTS: The neonatal dose and a first dose of BCG at school age protect against the incidence of severe COVID-19 in multivariate models, whose efficacies were 30 % (95 %CI:1-51) and 64 % (95 %CI: 22-84), respectively. The neonatal dose showed an effect on severe clinical progression of symptomatic COVID-19 disease in COVID-19 infected subjects 39 % (95 %CI:11 % - 58 %). CONCLUSION: Even 23 years after BCG vaccination and revaccination of school-age children our results suggesting a protective effect of BCG first dose against incidence of severe COVID-19 in infected individuals, a smaller effect of the neonatal dose and no effect of the second dose at school age.